1. Field of the Invention
The present invention relates to an apparatus with a rotational body that is rotationally driven in a fluid-filled housing. The invention relates especially to an X-ray radiator of the type having a cathode and anode that are mounted in a vacuum tube in a spatially fixed manner in relation to the vacuum tube, the vacuum tube being rotatably supported as a rotational body in a coolant housing, and having a stationary deflection system for lateral deflection of an electron beam directed from the cathode to the anode. An X-ray radiator of this type is normally designated as a “rotating piston radiator”.
2. Description of the Prior Art
X-ray radiation is normally generated by striking an anode with an electron beam emanating from a cathode. The cathode and the anode are mounted in a vacuum tube. Normally, an X-ray radiator is equipped nowadays with an anode that rotates under the incident electron beam in order to avoid a stationary focal spot relative to the anode. The focal spot, i.e., the point at which the electron beam strikes on the anode surface, is displaced, from the viewpoint of a coordinate system rotating with the anode, along a circular path over the anode surface. In this manner, the heat produced upon incidence of the electron beam is distributed comparatively uniformly on the anode surface, so material overheating in the cathode spot is counteracted.
In a “rotating piston radiator” of this type, the cathode and the anode are joined in a rotationally fixed manner to the vacuum tube and are rotated along with it. Here, the relative movement of the focal spot with respect to the anode surface is produced by the electron beam being deflected along a spatially fixed lateral direction out of the rotational axis of the vacuum tube, and thus it strikes the anode at a distance from the rotational axis of the rotating anode.
An X-ray radiator of the type described above is known, for example, from German Utility Model 87 13 042. The vacuum tube of this known X-ray radiator is surrounded by a protective housing filled with insulating oil and is rotatably supported therein around its center axis. The insulating oil (which acts simultaneously as a coolant) flows through the protective housing and thus enables a dissipation of the heat that arises during the operation of the X-ray radiator. A disadvantage of this known X-ray radiator is the friction losses of the coolant that is put into rotation as the vacuum tube rotates. To compensate for these friction losses, a drive power, which is not insignificant, is required which is mostly converted in a wasteful manner into heat and an acceleration of the coolant.
In order to reduce the friction losses within the coolant, in X-ray radiators known from U.S. Pat. No. 6,364,527 and U.S. Pat. No. 5,703,926, the vacuum tube is accommodated in a coolant housing which is rotated along with the vacuum tube. Due to the fact that the vacuum tube, the coolant housing as well as the coolant disposed therebetween rotate at the same or a similar angular speed, the friction loss within the coolant is reduced to a small level. A coolant container that rotates along with the vacuum tube, however, can be implemented only in a comparatively expensive manner, particularly since it must be provided with sealed bearings. Moreover, there is a disadvantage that, due to the rotating coolant housing, additional centrifugal forces arise which can counteract a fast rotation of the vacuum tube.
U.S. Pat. No. 6,213,639 discloses an X-ray radiator having a cathode and anode that are mounted in a vacuum tube in a spatially fixed manner in relation to the tube wherein the vacuum tube is rotationally driven in a coolant housing and a stationary deflection system is provided for lateral deflection of an electron beam directed from the cathode to the anode. Between the vacuum tube and the coolant housing, a coolant directing body attached on the coolant housing is provided.
The above-described problem is not limited to X-ray radiators. An undesired friction loss of the described type occurs in every rotational body driven in a fluid bath. Rotational bodies of this type are used, for example, in turbine technology, drive technology and cooling technology.